The present invention relates to a method and a device for measuring interface levels in separation tanks, and methods and devices for measuring concentrations/parts of a conductive fraction in flowing multi-phase mixtures, and especially where the fluids are immiscible.
The inventions have particular relation to the oil industry where immiscible phases of hydrocarbons (oil and gas) and water are being handled, as there may be present salts (give salinity to the water) in the water fraction, and greater or smaller amounts of solid particles, such as sand. The invention can be applied when one handles flowing mixtures of such fluids and wishes to know relative compositions; or in connection with separation facilities where for example oil and water are to be separated from each other.
During production of crude oil, water and gas are separated from the oil onboard the production platforms with the aid of separation tanks that function according to the principle of gravitation. The process water lies at the bottom of the tank. The next layer is an oil/water emulsion. Then comes crude oil alone, which higher up passes into foam which eventually passes into pure hydrocarbon gas. To optimise the separation process, it is necessary to be able to measure the levels of the different layers. There are many devices for measuring the height of the different interface levels, but they all have their limitations and only a few can measure the heights of an emulsion layer and a foam layer.
It is an aim of the invention to provide a new method and device to measure the levels of the different layers in a separation tank.
Furthermore, it is an aim of the invention to provide a new method and device for measuring concentrations/parts of a conductive fraction in flowing multi-phase mixtures, and particularly where the fluids are mutual immiscible. In one embodiment, the invention provides a method for measuring interface levels between fluids. In this method, a variable magnetic field is established in one of the fluids whereby a counter-flowing magnetic field is established as a function of the properties of the fluid with respect to the portion of the conductive fraction in the fluid and the conductivity of the fraction. Further, the properties of the conductive fluid are registered by registering the prominent impedance or resonance frequency of the system. One or more interface levels that are present are then determined by corresponding registration in different fluid layers at different height levels and in the existent interface layers followed by mutual comparison of these properties.
In another embodiment, a method is provided for measuring concentrations of water in a flow of an oil, gas and water mixture. In this method, a flow of the mixture of oil, gas and water is directed through a pipe having an excitation coil and a detector coil around the pipe in axially spaced relation to the excitation coil. Further, the coils have a different resonant frequency from each other. In accordance with the method, an alternating voltage is applied to the excitation coil at a frequency of up to 20 MHz in order to induce a variable magnetic field in the mixture. The resulting detector voltage is then registered in the detector coil as a measure of the electrical conductivity of the water in the mixture independently of the fraction of the oil and gas in the mixture. Thereafter, the resultant detector voltage is compared against a calibration value in order to determine the concentration of water in the mixture.
In another embodiment, a flow of a mixture of oil, water and gas is passed through a pipe having a pair of excitation coils and a detector coil disposed about the pipe. In this embodiment, an alternating voltage is applied to one of the excitation coils at a frequency of up to 20 MHz while an alternating voltage of a different frequency of up to 20 MHz is applied to the other excitation coil. The resultant induced voltage in the detector coil thus contains two frequencies and are registered as a measure of the electrical conductivity of the water in the mixture independently of the fractions of oil and gas in the mixture. The amplitudes and frequencies of the induced voltage are then detected and compared to a mathematical model in order to determine the concentration of water in the mixture and the conductivity of the water in the mixture.
By plotting the induced voltage (impedance) as a function of the concentration of the water in the flowing mixture, any abrupt decline in the induced voltage with increasing water concentration may be determinative as a boundary layer between a water-continuous phase containing oil droplets in water and an oil-continuous phase containing water droplets in oil.